The present invention relates to transistor protection circuitry. The invention is suitable for manufacture in integrated circuit form and can be used in many types of circuits which provide an output current that requires limiting.
Current limit circuits are employed to protect devices and loads from excessive damaging currents. They fall into two main classifications--direct current limit schemes and indirect or approximate current limit schemes. The present invention is directed to an approximate current limit circuit. An approximate current limit circuit is comprised of:
an output transistor of a particular conductivity type made up of a collector, base and emitter, PA1 a driver transistor of the same conductivity type made up of collector, base and emitter configured so that the emitter of the driver transistor couples to the base of the output transistor, PA1 a current sensing transistor of the same conductivity type made up of collector, base and emitter and connected so that the base and emitter of the current sensing transistor is tied to the base and emitter, respectively, of the output transistor so that the collector current of the current sensing transistor is a measure of the current flowing in the output transistor, and PA1 circuitry for limiting the collector current of the output transistor based on the collector current of the sensing transistor.
Circuits which use the approximate current limit scheme as described above are known in the art. For example, U.S. Pat. No. 4,330,757 (Fukuya et al.) discloses a circuit in which the current through the output transistor is measured by means of a current sensing transistor whose base-emitter junction is arranged in parallel with that of the output transistor, the collector current of the current sensing transistor being a fraction of the collector current of the output transistor. By means of a current mirror, the collector current of the current sensing transistor is reflected to a resistance element which is arranged in parallel with the base-emitter junction of a clamping transistor, the collector of the clamping transistor being connected to the base of the driver transistor. In the case of an excessive collector current of the current sensing transistor the clamping transistor is driven into conduction so that, what would otherwise be a part of the base current of the driver transistor is drained via the clamping transistor.
Also, U.S. Pat. No. 4,870,533 (Bahlmann) discloses a transistor protection circuit using the approximate current limit scheme. This patent discloses a current limit means which relies on a reference current and a current comparison. When the collector current of the current sensing transistor is substantially equal to the reference current, drive current to the output transistor is drained.
It is desirable in approximate current limit schemes to avoid the use of PNP transistors. Such transistors introduce instability in the current limit loop due to phase shifts caused by low cutoff frequencies associated with these transistors.